MEMS devices, in particular inertial sensors such as accelerometers and angular rate sensors or gyroscopes, are being used in a steadily growing number of applications. As the number of these applications grows, the greater the demand to add additional functionality and more types of MEMS into a system chip architecture. Due to the significant increase in consumer electronics applications for MEMS sensors such as optical image stabilization (OIS) for cameras embedded in smart phones and tablet PCs, virtual reality systems and wearable electronics, there has been a growing interest in utilizing such technology for more advanced applications which have been traditionally catered to by much larger, more expensive and higher grade non-MEMS sensors. Such applications include single- and multiple-axis devices for industrial applications, inertial measurement units (IMUs) for navigation systems and attitude heading reference systems (AHRS), control systems for unmanned air, ground and sea vehicles and for personal indoor and even GPS-denied navigation. These applications also may include healthcare/medical and sports performance monitoring and advanced motion capture systems for next generation virtual reality. These advanced applications often require lower bias drift and higher sensitivity specifications well beyond the capability of existing consumer-grade MEMS inertial sensors on the market. In order to expand these markets and to create new ones, it is desirable and necessary that higher performance specifications be developed. It is also necessary to produce a low cost and small size sensor and/or MEMS inertial sensor-enabled system(s).
Given that MEMS inertial sensors such as accelerometers and gyroscopes are typically much smaller than traditional mechanical gyroscopes, they tend to be subject to higher mechanical noise and drift. Also, since position and attitude are calculated by integrating the acceleration and angular rate data, respectively, noise and drift can lead to growing errors. Consequently, for applications requiring high accuracy, such as navigation, it is generally desirable to augment the six-degree-of-freedom (6DOF) inertial capability of MEMS motion sensors (i.e., three axes of acceleration and three axes of angular rotation) with other position- and/or orientation-dependent measurements. By way of example, barometric pressure measurements can provide additional information about altitude, while magnetic field measurements can provide additional information about position on the Earth's surface and motion relative to the Earth's magnetic field. Thus, for MEMS inertial sensor systems, as well as other MEMS sensor systems, it is attractive to integrate more types of sensors onto a single chip.
The MEMS devices that measure these parameters include a MEMS mechanical element (e.g. proof mass, pressure-sensitive membrane, or magnetic transducer) that is free to move in response to a particular measured or stimulus. Additionally, since MEMS transducers are by design sensitive to some environmental influences, the packaging surrounding a MEMS transducer should protect it from undesired environmental influences. Thus the MEMS package surrounding the transducer should provide a hermetic, and in some cases a vacuum, environment while at the same time enabling electrical contact between the enclosed sensors and their corresponding IC electronics. In the past this has been accomplished by packaging the MEMS and IC side by side, fabricating the MEMS directly on the IC, or stacking the MEMS and IC, followed by attaching the MEMS and IC to a package substrate, protecting the MEMS with a non-functional silicon or glass cap, using wire bonds to make electrical connection to the IC and package substrate, and covering the substrate with a molded plastic cap. This chip-scale packaging adds considerable expense to the final device and makes chip stacking for 3DIC applications difficult, if not impossible.
Efforts have been made to include electrical feedthroughs through the cap over MEMS sensors, such as copper-filled or polysilicon-filled through-silicon-vias (TSVs). These TSVs consist of holes etched in the silicon that are lined with an insulator (e.g., thermal silicon dioxide), and then filled with a conductor (e.g., copper or polysilicon). In order to completely fill the TSV while limiting the diameter of the holes and, thus, the thickness of the fill material, the depth of the holes generally does not exceed about 100 micrometers (μm). The thickness of the MEMS cap is thus also limited to around 100 micrometers, rendering it susceptible to flexing due to pressure differences between the inside and the outside of the package, and also to external mechanical and thermal stresses. This flexing can cause delamination and cracking in the thin film layers of the TSVs which, in turn, can lead to leaks of air and moisture into the package and destroy or degrade its hermeticity.
Also, the performance of MEMS sensors generally depends on their operating environment, particularly the pressure environment. For example, resonant devices such as gyroscopes, silicon clocks and magnetometers typically operate at low or even vacuum pressures to minimize air damping and improve the quality factor of the resonance. Accelerometers, on the other hand, generally require some air damping to lower their ringing response to external impulse forces and enhance the response to the slowly varying accelerations of interest. Pressure sensors and microphones generally require access or exposure to the ambient pressure environment outside the sensor and may contain in their interiors gas of either high or low pressure, depending upon the application. In addition, for wafer level packaging of MEMS sensors, the pressure inside the sensor is typically determined by the ambient pressure at the time of wafer bonding. Thus, every sensor on the wafer is generally sealed in an environment at the same pressure. In order to integrate different sensors requiring different ambient pressures on the same chip, it may be needed or desirable to provide for each sensor an individual micro-environment at a desired pressure.
Accordingly, challenges remain in the development of methods for packaging MEMS devices. In light of the preceding, there is a need for an improved MEMS device and related fabrication process.